Biofilms are microbial communities, held together and to a substrate by a gel-like material. Biofilms cover all marble monuments. The biofilms that are considered visually unattractive are black; the black color is melanin, a pigment produced in response to UV radiation. Conservators are often tasked with “cleaning,” or “removing” a melanin-producing biofilm. To better understand these biofilms and concerns for their removal, the National Park Service’s Historic Architecture, Conservation, and Engineering Center executed a series of cleaning tests for long-term monitoring (15 years ago and 3 years ago); collaborated with the Center for Biofilm Engineering, Montana State University, and the Mathematics Department, Temple University on a National Science Foundation grant; and collaborated with students in the Biotechnology Lab, Thomas Jefferson High School for Science and Technology, Arlington, VA. Results from research and tests thus far conclude: 1. Biofilms are far more complex than one can imagine. 2. The bio-receptive surface of eroded marble is the perfect substrate for biofilms. 3. Stressing biofilms (many cleaning treatments) will produce immunities and a biofilm more resistant to that stress when it returns. 4. All brands of products containing quartenary ammonium compounds (quats) used for cleaning tests behaved the same. 5. Quats can be used as a diagnostic tool to determine the presence of biofilms, whether they contain melanin or not. 6. There is a relationship between biofilms, moisture, and salt deliquescence on a marble surface. Biofilms might be keeping surface salts in solution. 7. Microorganisms categorized as “extremeophiles” are found on white marble monuments in Washington, DC. 8. DNA sequencing of biofilm samples taken from a marble surface before and after cleaning tests is a way to understand success—or not—of cleaning. Collaborations continue, tests continue, observations continue, and research continues. Assumptions of biofilm activity and relationships to marble need to be constantly questioned. There is no solution to keeping eroded white marble monuments white, but with the efforts of many institutions, individuals, and industry, working together, we will learn more about biofilms and may even find a solution to keeping melanin-producing biofilms at bay.

Judith M. Jacob is a Senior Conservator with the National Park Service (NPS), Northeast Region, Historic Architecture, Conservation, and Engineering Center. She provides conservation support for NPS buildings, structures, and monuments. Her responsibilities include planning and administrating... Read More →